Now, the curious part about this wet plate process is that it is slow. The compounds are not very sensitive compared with the modern compounds. In the second place it is essential to use it wet. If you took the plate out of the silver bath where you sensitized it and washed off the nitrate of silver adhering to it and put the plate in the camera you would not get a picture. The silver nitrate is essential to the production of the picture. It acts in this way: Where the light has acted upon the sensitive silver compounds and you proceed to develop the picture, when you mix the sulphate of iron and pour the developer upon the plate, as the iron comes in contact with the nitrate of silver, with which the plate is wet, it produces metallic silver, which adheres to those parts of the picture which have been acted upon by the light. That seems to be the philosophy, because if you wash the nitrate off you cannot develop a picture upon such a plate. Now, this process of photography revolutionized the daguerreotype, revolutionized photography and the daguerreotype became obsolete. I think it displaced the daguerreotype in three years. This process was such an advantage--collodion was such a nice substance to work with--that it revolutionized the photography of those days, and the daguerreotype fell out of existence. Now, when you take into consideration the time that people had to sit for their pictures--five or six minutes--you can conceive how hard it was to keep still. They had such queer contrivances to keep the head straight, they screwed you up in various positions, and this was particularly exasperating where they had to take pictures requiring a good deal of time. Dr. Draper, who took some of these daguerreotypes, and who I believe was the first photographer of these pictures, desired to take a photo of his estimable lady. His studio was in the old University Building in Washington Square. I believe Mrs. Draper had to sit twenty minutes for that picture. In order to produce the best effect he had a tank made in the top of the laboratory so as to produce a blue light. Mrs. Draper was very patient while he was at work with this, and unfortunately, Dr. Colton tells me, the result was two pictures on the same plate. I should think it would. That was the first effort ever made to take the human face with the daguerreotype. Of course, with all that paraphernalia, with that slowness of action, anything that worked within a minute was considered wonderful, and that was practically what happened when Scott Archer discovered collodion.